Let's fetch a simple map on which we can display our points
Here we are centering the map on San Francisco & using the stamen toner-lite basemap.
sf_center_point <- c(lon=-122.445144, lat=37.769335 )
sf_map <- get_map(sf_center_point, source="stamen", maptype="toner-lite")
## Map from URL : http://maps.googleapis.com/maps/api/staticmap?center=37.769335,-122.445144&zoom=10&size=640x640&scale=2&maptype=terrain&sensor=false
## Map from URL : http://tile.stamen.com/toner-lite/10/162/394.png
## Map from URL : http://tile.stamen.com/toner-lite/10/163/394.png
## Map from URL : http://tile.stamen.com/toner-lite/10/164/394.png
## Map from URL : http://tile.stamen.com/toner-lite/10/162/395.png
## Map from URL : http://tile.stamen.com/toner-lite/10/163/395.png
## Map from URL : http://tile.stamen.com/toner-lite/10/164/395.png
## Map from URL : http://tile.stamen.com/toner-lite/10/162/396.png
## Map from URL : http://tile.stamen.com/toner-lite/10/163/396.png
## Map from URL : http://tile.stamen.com/toner-lite/10/164/396.png
## Map from URL : http://tile.stamen.com/toner-lite/10/162/397.png
## Map from URL : http://tile.stamen.com/toner-lite/10/163/397.png
## Map from URL : http://tile.stamen.com/toner-lite/10/164/397.png